The continent/ocean transition of the Lofoten volcanic margin, N. Norway

Abstract

A detailed model for the continent/ocean transition of the Lofoten passive volcanic margin, N. Norway, has been obtained by use of seismic wide-angle data from Ocean Bottom Seismographs (OBSs), combined with results from other geophysical data. The OBS experiment across the transition zone involved dip- and strike-profiles, as well as a profile crossing the zone obliquely. The modelling reveals that this oblique profile is a key profile to the understanding of the transition zone, since detailed crustal models are difficult to obtain by use of the dipprofiles due to the presence of strong lateral variations in structures and seismic velocities. The Early Cenozoic extension episode on the outer Lofoten Margin ended with extensive submarine volcanism accompanied by intrusions into the stretched continental crust and emplacements of magmatic bodies into the lower crust. The high magmatic activity can be explained by increased asthenosphere temperatures caused by the proximity to the Iceland plume. From the seawarddipping-reflectors (SDRs) to the Vøring Escarpment (VE) a landward decrease in the upper crustal P-wave velocity from about 6.7 to 6.0 km/s is observed from the OBS data. This lateral velocity variation is interpreted to be caused by a gradual landward decrease in the amount of high-velocity “oceanic” intrusions into the stretched continental crust. The same landward decrease in velocity is also observed in the lower crust; from about 7.3 to 6.8 km/s. The modelling suggests that the high-velocity lower crustal body terminates landwards close to the VE. This lower crustal body is interpreted as magmatic underplating of the crust associated with the Iceland plume. The Mono (upper mantle) velocity is inferred to be 8.2 km/s from the shelf to the SDRs, and 7.9 km/s seward of the SDRs. This lateral velocity variation suggests that the upper mantle is of continental origin landward of the SDRs, and of oceanic origin from the SDRs and westwards. This observation indicates that the lower crustal magmatic material is intruded laterally into the ductile lower crust from the oceanic side. It is concluded that the transition from stretched continental crust with non-observable amounts of magmatic intrusions, to oceanic crust takes place over an about 50 km wide zone from the VE to the SDRs on the Lofoten Margin.